1. Field of the Invention
The invention relates to a radio communication terminal which uses a plurality of discrete frequencies at the same time.
2. Description of the Related Art
In order to increase the communication speed of consumer radio communication devices, movements to change the assignment and usage of frequencies have begun. Previously, specific bands are exclusively assigned for specific applications. However, recently, some bands have been opened up to a plurality of applications without licenses. Furthermore, a very wide band has been opened to ultra-wideband (UWB) system, which is limited to low-power, short-range communications so as to be used together with other applications.
As a future system, studies about an idle frequency detection type cognitive radio system which operates to output a radio wave of a given frequency if it is confirmed that the frequency is not used, and to stop the communication when a system having priority to that frequency begins to transmit a radio wave have begun.
Since the cognitive radio system uses frequencies while avoiding those which are being used by a system having priority (to be referred to as a primary system hereinafter), usable frequencies are in a so-called vermiculate state. The bandwidth that can be used contiguously varies depending on frequencies.
In order to assure a bit rate required for high-speed communications, since a large bandwidth in total need be assured, the system cannot help but use discrete frequency bands in the vermiculate state. Usable frequencies change frequently upon start and stop of use of primary systems, and the system need to transmit and receive signals of different bandwidths at discrete frequency bands at the same time. In such case, a transmitter and receiver are more likely to digitally multiplex and separate a plurality of signals within a wide band using a set of an analog unit, analog-to-digital converter, and digital-to-analog converter, and to perform modulation and demodulation in a digital unit. Upon simultaneously making analog-to-digital and digital-to-analog conversions of signals within a large bandwidth, the analog-to-digital and digital-to-analog converters require large bandwidths. Currently, the clock speeds of these devices are exponentially increasing. However, it is still difficult to manufacture devices using clocks on the GHz order at low cost and with high precision and high performance.
In addition to the problem of the speed, due to difficulty in filter configuration, the transmitter and receiver may perform analog-to-digital and digital-to-analog conversions of in-phase (I) signal components and orthogonal phase (Q) signal components at a half clock speed and may add and separate I and Q components in an analog unit (for example, see JP-A 5-14424 [KOKAI] FIG. 6, [0003], [0004]).
As a system which simultaneously uses a plurality of frequencies, for example, a general frequency division multiplex system is available. Also, a special frequency division multiplex system which inverts an identical signal in the frequency domain and allocates them to different frequencies is available (for example, see JP-A 2001-267997 [KOKAI]). As a system in which one terminal simultaneously uses two or three frequencies, for example, a system using subcarrier AM modulation is available.
In the transmitter with such configuration, I and Q components generated by the digital unit, the phases of I and Q signal need be precisely orthogonal to each other and be added to have equal gains upon adding in the analog unit. Or else, intended signals cannot be generated upon adding, thus posing a problem of imbalance. In a conventional radio communication, a problem posed due to imbalance is that within the self band. That is, the signal to interference power ratio of the self signal deteriorates, thus impairing communication performance (for example, see JP-A 7-327059 [KOKAI]). As described in JP-A 7-327059 (KOKAI), efforts for achieving better reception performance by correcting such deterioration in a receiver are being made. Furthermore, various measures for the transmitter to reduce the imbalance as much as possible have been proposed.
The cognitive radio system uses frequencies to fill the niche of frequencies used by the primary system. When the digital-to-analog converter converts signals in a plurality of frequency bands into analog signals independently for I and Q, and the analog unit adds these signals while the imbalance between I and Q still remains, spurious components may be generated at the frequencies of the primary system. The spurious components may interfere with the primary system depending on their magnitudes. It is difficult for a cognitive terminal, which is configured to simultaneously digital-to-analog convert a very large bandwidth, to reduce the imbalance with respect to the very wide band.